1. Technical Field
The present invention relates to a direct-sequence CDMA (Code Division Multiple Access) (DS-CDMA) communication system, and in particular, to a data transmitter and receiver of a DS-CDMA communication system, which prevent on-off or serious amplitude shift of a transmission signal and facilitate recovery of data and clock signals.
2. Related Art
Spread spectrum communication is described in many publicly available documents, such as Spread Spectrum Communications by Marvin K. Simon et al. (Computer Science Press, 1989), Spread Spectrum Communications Handbook by Marvin K. Simon et al. (McGraw-Hill, 1994), Spread Spectrum System With Commercial Applications by Robert C. Dixon (John Wiley & Sons, 1994), U.S. Pat. No. 5,431,395 for Data Recovery Technique For Asynchronous CDMA Systems issued to Bi, U.S. Pat. No. 5,361,276 for All Digital Maximum Likelihood Based Spread Spectrum Receiver issued to Subramanian, U.S. Pat. No. 5,400,359 for Spread Spectrum Communication System And An Apparatus For Communication Utilizing This System issued to Hikoso et al., U.S. Pat. No. 5,414,728 for Method And Apparatus For Bifurcating Signal Transmission Over In-Phase And Quadrature Phase Spread Spectrum Communication Channels issued to Zehavi, U.S. Pat. No. 5,416,797 for System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System issued to Gilhousen et al., U.S. Pat. No. 5,546,420 for Methods Of And Devices For Enhancing Communications That Use Spread Spectrum Technology By Using Variable Code Techniques issued to Seshadri et al., U.S. Pat. No. 5,550,810 for Direct Sequence Code Division Multiple Access (DS-CDMA) Communication System And A Receiver For Use In Such A System issued to Monogioudis et al., U.S. Pat. No. 5,583,835 for Synchronous CDMA Transmitter/Receiver issued to Giallorenzi et al., U.S. Pat. No. 5,596,600 for Standalone Canceller Of Narrow Band Interference For Spread Spectrum Receiver issued to Dimos et al., U.S. Pat. No. 5,646,964 for DS/CDMA Receiver For High-Speed Fading Environment issued to Ushirokawa et al., U.S. Pat. No. 5,694,388 for CDMA Demodulator And Demodulation Method issued to Sawahashi et al., and U.S. Pat. No. 5,734,647 for CDMA Communication System In Which Interference Removing Capability is Improved issued to Yoshida et al. As is well known, spread spectrum signals are used for (i) combating interference, (ii) transmitting at very low power to avoid detection/interception, and (iii) multiplexing one channel over many users. Spread spectrum signal processing is characterized by expanding the bandwidth of a message signal, transmitting the expanded signal, and recovering the message signal by remapping the spread spectrum into the original bandwidth. The method of spread spectrum communication includes direct sequence, frequency hopping, time hopping and a hybrid combing two or more of these. Direct sequence spreads the spectrum by multiplying data and the pseudo-random noise (PN) code having a chip rate considerably higher than data rate, of which circuitry can be implemented relatively easily as compared with those used in other methods. Use of different PN codes allows multiple access in the same frequency band. Such multiple access is called Code Division Multiple Access (CDMA) or Spread Spectrum Multiple Access (SSMA).
A radio communication system using direct sequence spread spectrum is commonly known as a Direct Sequence Code Division Multiple Access (DS-CDMA) systems, according to TIA/EIA standard IS-95. Individual users in the system use the same radio frequency (RF) but are separated by the use of individual spreading code. In a spread spectrum communication system, downlink transmissions include a pilot channel and a plurality of user traffic channels. The pilot channel is coded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations. The pilot channel is used to provide timing and carrier phase synchronization in the receiver of a mobile station, and estimation of the gain of the channel and the phase shift imposed by the channel.
Exemplars of a data transmitter and receiver in a DS-CDMA communication system using a pilot channel and a plurality of user traffic channels are disclosed in Korea Application No. 94-20801 for Data Transceiver in Spread Spectrum Communication System Using Pilot Channel and Korea Application No. 94-30497 for Data Transceiver in Spread Spectrum Multiple Access Communication System Using Pilot Channel, U.S. Pat. No. 5,675,608 for Synchronous Transmitter And Receiver Of Spread Spectrum Communication Method issued to Kim et al., U.S. Pat. No. 5,712,869 for Data Transmitter And Receiver Of A Spread Spectrum Communication System Using A Pilot Channel issued to Lee et al, all are assigned to the instant assignee and incorporated by reference herein. Based upon our studies of many contemporary data transmitters and receivers of a DS-CDMA communication system using a pilot channel, we have observed that the receiver often incorrectly determines that a spread spectrum signal is on/off due to the simultaneous zero crossing of an in-phase (I) and quadrature-phase (Q) channel data. As a result, recovery of data and clock signal has been entirely satisfactory.